Voltage controlled oscillator, VCO, for PLLs
- an overview of the various types of voltage controlled oscillator, VCO,
used in phase locked loops, PLLs and frequency synthesizers
Within a phase locked loop, PLL, or frequency synthesizer,
the performance of the voltage controlled oscillator, VCO is of paramount
importance. In order that the PLL or synthesizer can meet its full specification
a well designed oscillator is essential. Designing a really high performance
voltage controlled oscillator, VCO, is not always easy as there are a number of
requirements that need to be met. However by careful design, and some
experimentation a good VCO design can be found.
VCO requirements
Just like any other circuit, with a VCO there are a number of
design requirements that need to be known from the beginning of the design
process. These basic requirements for the VCO will govern many of the decisions
concerning the circuit topology and other fundamental aspects of the circuit.
Some of the basic requirements are:<.p>
- Tuning range
- Tuning gain - tuning shift for a given tuning voltage change
- Phase noise (low phase noise)
These are some of the main requirements that need to be known
from the outset of the design of the VCO. The overall tuning range and the gain
are basic requirements that are part of the basic design of any PLL into which
the VCO may be incorporated. So too is the phase noise characteristic. As phase
noise is a basic parameter of any PLL or frequency synthesizer, so too is the
characteristic of the VCO, and low phase noise VCOs are often required. For
example the VCO performance may govern the overall design of the frequency
synthesizer or PLL, if a given phase noise performance is to be met.
VCO circuits
Like any oscillator, a VCO may be considered as an amplifier
and a feedback loop. The gain of the amplifier may be denoted as A and the
feedback as B.
For the circuit to oscillate the total phase shift around the
loop must be 360 degrees and the gain must be unity. In this way signals are fed
back round the loop so that they are additive and as a result, any small
disturbance in the loop is fed back and builds up. In view of the fact that the
feedback network is frequency dependent, the build up of signal will occur on
one frequency, the resonant frequency of the feedback network, and a single
frequency signal is produced.
Many oscillators and hence VCOs use a common emitter circuit.
This in itself produces a phase shift of 180 degrees, leaving the feedback
network to provide a further 180 degrees.
Other oscillator or VCO circuits may use a common base
circuit where there is no phase shift between the emitter and collector signals
(assuming a bipolar transistor is used) and the phase shift network must provide
either 0 degrees or 360 degrees.
Colpitts and Clapp VCO circuits
Two commonly used examples of VCO circuits are the Colpitts
and Clapp oscillators. Of the two, the Colpitts circuit is the most widely used,
but these circuits are both very similar in their configuration.
These circuits operate as oscillators because it is found
that a bipolar transistor with capacitors placed between the base and emitter
(C1) and the emitter and ground (C2) fulfils the criteria required for providing
sufficient feedback in the correct phase to produce an oscillator. For
oscillation to take place the ratio C1: C2 must be greater than one.
The resonant circuit is made by including a inductive element
between the base and ground. In the Colpitts circuit this consists of just an
inductor, whereas in the Clapp circuit an indictor and capacitor in series are
used.
The conditions for resonance is that:
f^2 = 1 / (4 pi^2 L C )
The capacitance for the overall resonant circuit is formed by
the series combination of the two capacitors C1 and C2 in series. In the case of
the Clapp oscillator, the capacitor in series with the inductor is also included
in series with C1 and C2.
Thus the series capacitance is:
Ctot = 1 / C1 + 1 / C2
In order to make the oscillator tune it is necessary to vary
the resonant point of the circuit. This is best achieved by adding a capacitor
across the indictor in the case of the Colpitts oscillator. Alternatively for
the Clapp oscillator, it can be the capacitor in series with the inductor.
For high frequency applications a circuit where the inductive
reactance is placed between the base and ground is often preferred as it is less
prone to spurious oscillations and other anomalies.
Choice of VCO active device
It is possible to use both bipolar devices and FETs within a
VCO, using the same basic circuit topologies. The bipolar transistor has a low
input impedance and is current driven, while the FET has a high input impedance
and is voltage driven. The high input impedance of the FET is able to better
maintain the Q of the tuned circuit and this should give a better level of
performance in terms of the phase noise performance where the maintenance of the
Q of the tuned circuit is a key factor in the reduction of phase noise.
Another major factor is the flicker noise generated by the
devices. Oscillators are highly non-linear circuits and as a result the flicker
noise is modulated onto VCO as sidebands and this manifests itself as phase
noise. In general bipolar transistors offer a lower level of flicker noise and
as a result VCOs based around them offer a superior phase noise performance.
VCO tuning
To make a VCO, the oscillator needs to be tuned by a voltage.
This can be achieved by making the variable capacitor from varactor diodes. The
tune voltage for the VCO can then be applied to the varactors.
When varactor diodes are used, care must be taken in the
design of the circuit to ensure that the drive level in the tuned circuit is not
too high. If this is the case, then the varactor diodes may be driven into
forward conduction, reducing the Q and increasing the level of spurious signals.
There are two main types of varactor diode that may be used
within a VCO: abrupt and hyper-abrupt diodes. The names refer to the junction
within the diode. The abrupt ones do not have a sharp a transition between the
two semiconductor types in the diode, and this affect the performance offered.
Hyper-abrupt diodes have a relatively linear voltage :
capacitance curve and as a result they offer a very linear tuning characteristic
that may be required in some applications. They are also able to tune over a
wide range, and may typically tune over an octave range with less than a 20 volt
change in tuning voltage. However they do not offer a particularly high level of
Q. As this will subtract from the overall Q of the tuned circuit this will mean
that the phase noise performance is not optimum.
Abrupt diodes, while not offering such a high tuning range or
linear transfer characteristic are able to offer a higher Q. This results in a
better phase noise (i.e. low phase noise) performance for the VCO. The other
point to note is that they may need a high tuning voltage to provide the
required tuning range, as some diodes may require a tuning voltage for the VCO
to vary up to 50 volts or slightly more.
Summary
The design of a VCO can be interesting and challenging.
Whether the aim is to design a low noise VCO, a low current VCO, a PLL VCO, or
one that will cover a wide tuning range there are many aspects that need to be
addressed. Often when a successful design has been obtained, it will slightly
modified to enable it to cover a wide range of similar applications.
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